The present invention relates to an optical system for an optical disc drive handling different types of optical discs having protective layers of different thickness.
There are several types of optical discs such as a digital versatile disc and a compact disc. The thickness of the protective layers, i.e., the transparent substrate covering a recording layer of the optical disc, depends on the type of the optical disc. For example, the protective layer of the compact disc (CD) is 1.20 mm thick while that of the digital versatile disc (DVD) is 0.60 mm thick. The protective layers having different thickness cause spherical aberrations of different amount. Therefore, an optical system for an optical disc drive that handles different types of optical discs should be designed by taking into account the difference of the spherical aberrations between each type of optical disc.
Since the recording density of the digital versatile disc is much higher than that of the compact disc, size of the beam spot to be formed on the digital versatile disc is required to be much smaller than that for the compact disc. The difference between the beam spot sizes requires the optical disc drive to utilize shorter wavelength laser beam for the digital versatile disc, i.e., 635-665 nm, than for the compact disc, i.e., 780-830 nm, since the beam spot size can be made smaller as the wavelength decreases.
Recently, it is proposed to employ an objective lens provided with a diffraction structure for the optical system of the optical drive handling different types of optical disc. The diffraction structure is a large number of concentric ring-shaped steps and converges each of the laser beams of different wavelengths onto the recording layer of the corresponding optical disc.
The diffraction structure is configured so that the spherical aberration generated thereby varies in accordance with the wavelength of the entering laser beam. In other words, the diffraction structure is designed so as to have wavelength dependency.
The spherical aberration caused by the objective lens provided with the diffraction structure mentioned above increases/decreases as the wavelength of the laser beam passed therethrough changes, and cancels the increase/decrease of the spherical aberration caused by the difference of the protective layer thickness of the optical disc to be handled. As a result, the optical system can form a beam spot of a desired size on the recording layer of each type of the optical disc.
Recently, a large amount of the objective lenses are made of plastic in order to reduce the cost. The spherical aberration caused by the plastic lens has a temperature dependency. That is, the spherical aberration varies with the temperature of the plastic lens since shape and refraction index of the lens changes as the temperature of the lens increases or decreases. Accordingly, if a plastic lens is employed for the objective lens, the lens should be designed by also taking into account the temperature dependency of the spherical aberration.
However, if the diffraction structure mentioned above is also designed to suppress the temperature dependency of the spherical aberration, both of the acceptable range of the laser beam wavelength and the acceptable range of the objective lens temperature become significantly restricted. The narrow wavelength range does not allow large wavelength deviation of the laser beam from the design wavelength and forces the use of high-end laser sources which are costly. The narrow temperature range restricts the environment in which the optical disc drive can be operated.
Therefore, there is a need for an optical system for an optical disc drive that handles different types of optical discs, which optical system is capable of correcting spherical aberration caused by the protection layer of the optical disc while allowing the laser beam wavelength deviating from the design wavelength.
There is also a need for an optical system for an optical disc drive that handles different types of optical discs, which optical system is hardly affected by temperature change even if plastic lens is employed as an objective lens thereof.